1. Field of the Invention
The present invention relates to a method for producing an elongated article article. More particularly, it relates to a method for producing an elongated sintered article in a form of wire, rod, tape or the like from a powder material.
The method according to the present invention is applicable to so called new ceramics, fine ceramics or sintered alloys which are difficult of shaping or molding by the conventional techniques such as wiredrawing, rolling or extrusion.
The method according to the present invention is advantageously applicable for manufacturing a superconducting wire.
2. Description of the Related Art
New ceramics, fine ceramics and sintered alloys possess a wide variety of functions, and hence their applications are spreading over a variety of industrial fields. They have a high thermal conductivity and a high resistance to chemicals. In addition to these inherent properties, some ceramics possess the other unique properties. In fact, transparent ceramics, histocompatible ceramics or electro-conductive ceramics are known. Such variety of the properties or functions come from such a fact that the ceramics are composed of large numbers of elements which can be combined in a wide range.
The term ceramics in this specification implies any sintered article produced through a solid reaction by a sintering technique from a powder material including inorganic powders and metal powders. The ceramics to which the present invention is applicable may be oxide type ceramics such as alumina, beryllia, zirconia, ferrites (MO,Fe.sub.2 O.sub.3), PLZT ((Pb,La)(Zr,Ti)O.sub.3) or the like, non-oxide type ceramics such as nitride, for example Si.sub.3 N.sub.4 or AIN, carbide, for example SiC or WC, boron carbide, carbide precipitating type cobalt-based alloy or the like. Some of them are often called sintered alloys.
Alumina have been one of very popular ceramics used in machine parts and now is used as a substrate material for IC. Silicon carbide and silicon nitride possess high abrasion-resistance and strength in a high temperature range, so that they are used as machine parts used in an internal combustion engine or in a high temperature conveyer machine. Tungsten carbide and the carbide precipitating type cobalt-based alloy possess a very high hardness and stiffness, so that they are used as a cutting tool or a printer head or the like.
However, machining or cutting of these materials are very difficult to perform because of their hardness which is one of their inherent properties. In fact, it is very difficult to cut the sintered bodies of these materials by ordinary cutting tools, so that they have been machined only by electro spark machining technique or diamond cutting technique. In other words, they are impossible or difficult to be deformed or shaped into elongated articles such as tape, wire or pipe by the conventional plastic deformation techniques such as rolling or wire-drawing.
In the conventional technique for producing an elongated article of ceramic, a powder material is molded into a compact at first and then the compact is baked in a sintering furnace in order to minimize a cutting work after a sintering stage. For, example, a ceramic shaft has been produced by such steps that a powder material is molded into a rod having a rectangular cross section in a press and then the resulting molded article is shaped in a trimming step before the press-molded article is passed to a sintering furnace. This conventional technique, however, is not economical because of poor productivity and waste of expensive powder material and also is not suitable for a continuous operation because an elongated article having a long length can not be produced.
In another conventional method for producing a ceramic article, the powder material is blended with organic binder to prepare a paste which can be used in a moulding or shaping stage. Then, the paste molded in a form of rod or tape is heated to remove the binder and then sintered finally. This process much better than the above-mentioned press molding technique but it is difficult to remove the organic binder completely during the intermediate heating stage and hence remain in the finally sintered article, resulting in a cause of defects of the product which will lower the strength and the resistance to flexion.
In the case of sintered metallic alloys such as carbide precipitating type cobalt-based alloy, it is also possible to adopt special techniques such as (i) centrifugal casting process, (ii) rotary spinning process in water and (iii) metal plating process in addition to the above-mentioned techniques for ceramics. However, it is difficult to produce an elongated article of high quality by the centrifugal casting process. The maximum length of a wire produced by the process is limited to 50 cm in case of a diameter of 2 mm. In the case of the rotary spinning process, it is difficult to control precisely a diameter of a wire to be produced and the maximum diameter of the wire is limited to 1 mm. The metal plating process is used for diffusing metal such as Co, W, Cr or the like into a carbon fiber. However, this technique is not only applicable to the other metals but also is poor in productivity.
Conclusively, it is impossible or difficult to manufacture elongated ceramic articles of high quality economically by the conventional techniques, and hence there remains a strong demand to establish an industrial process for manufacturing elongated articles.
The present inventors proposed a unique process for manufacturing an elongated article by the steps comprising filling a metal pipe with a material powder of ceramic, performing plastic deformation of the metal pipe filled with the ceramic metal powder to reduce the cross section of the metal pipe, and then subjecting the deformed metal pipe to heat-treatment to sinter the ceramic material powder filled in the metal pipe in a pending Japanese patent application No. 62-121,733.
This process itself is satisfactory. However, we found such a trouble in this process that, if the pipe is made of a metal such as aluminum whose melting point is low, the metal pipe fuses and dopes off the compacted powder during the sintering stage, resulting in that the metal pipe loses its function as a support. In fact, the sintered wire is apt to break if the support or metallic sheath does not exist, since the sintered mass produced by the process is fragile.
An object of the present invention is to overcome the abovementioned problems of our previous process and to provide an improved process for producing an elongated article covered by a metallic sheath with high quality, reduced loss and higher productivity.
Another object of the present invention to apply the method of the present invention for a production of a superconducting wire of compound oxide.